Some valve trains are selectively adjustable to vary the amount of valve travel. Typically, such valvetrains are selectively adjustable between a low-lift mode, in which the valvetrain causes an engine valve to open a first predetermined amount, and a high-lift mode, in which the valvetrain causes the valve to open a second predetermined amount that is greater than the first predetermined amount. Alternatively, the low-lift mode may be a zero-lift mode configured to allow valve deactivation.
Selectively adjustable valvetrains may include a plurality of two-step rocker arms that engage an engine valve and are pivotable in response to cam motion to lift the valve. The two-step rocker arm is hydraulically actuatable to engage either the low-lift mode or the high-lift mode. Lash adjusters are used to accommodate for build variation and wear in a valvetrain assembly. Lash adjusters are also typically configured to transfer pressurized hydraulic fluid to actuate the two-step rocker arms and thereby control the engagement of the low-lift and high-lift modes.
Traditionally, the transfer of pressurized fluid to the lash adjusters has been achieved by using a cylinder head having a complex system of fluid supply passages that enable pressurized fluid to communicate with the lash adjusters, which are supported in the cylinder head. Cylinder heads with such an integrated hydraulic system are necessarily specific to each engine family and entail numerous production steps such as casting, boring, and finishing the network of channels provided in the cylinder head. Additionally, packaging the fluid supply passages in the cylinder head is difficult because of the limited available space, and a compact fluid supply passage design is therefore preferable.